That is true. In Enterprise Arik Soong and his Augment "children" discussed how the Botany Bay was just some legend that had no evidence to ever substantiate it. Soong thought the story of the Botany Bay and Khan was complete bullshit.

Until Kirk and his Enterprise found the ship over 100 years later Khan had been thought to be long dead, possibly as far back as his defeat and exile at the end of the Eugenics Wars.